It doesn't make any sense because even if the code was in the game, it's not for access. The ESRB will rate game areas and game content that can be accessed by any gamer, without using any kind of mods. Modding is the same as hacking in this case and well, the case just won't hold up because of that. Rockstar could've left whatever code they wanted to in the game as long as it doesn't violate people's privacy (spyware, etc..), as long as the code never runs, it's as good as a line of comments in the source..
Something which was unlocked in the game by hacking into it makes it look like you were specifically looking for some kind of easter egg (hidden code-wise) or programmer message. Sure it says sick things about the geeky programmers of GTA, but other than that, it does no damage. That particular 'area' of the game isn't accessible without hacking/modding.
It's the same as saying you reverse-engineered a zelda cart and found nude zelda models, and that could be perfectly viable since the game engine might use a nude model and then render the clothes onto it. Sure, it was a sick joke by the programmers, but it wasn't meant for the public anyway, Rockstar wasn't wrong in leaving it in because they took adequate measures to make sure no gamers (not hackers..) could access it. Thus, the ESRB rating was according to what they saw in the game, pretty natural. They couldn't get at the nude stuff either..
Who knows what kind of code is there in all of our programs ? We only get binaries, there could be all sorts of sex scene avi's in our programs

. Hardly a reason to sue over as long as your program works okay..